A captive seal is one in which a sealing material is completely confined and compressively loaded beyond its yield point. The sealing material substantially completely fills the seal cavity in which it is positioned. The compressive loading is sufficient to cause the sealing material to flow into the minute crevices and surface irregularities of the wall of the cavity to form a tight seal. One advantage of captive seals is that the surfaces which confine the seal need not have a good surface finish. Captive seal constructions are shown, for example, in U.S. Pat. Nos. 3,572,735 and 3,594,022.
In using a captive seal, it is essential that the sealing material be highly compressively loaded into the surface to be sealed. In the absence of such loading, the sealing material will not engage, or will not engage with sufficient pressure, the surface to be sealed. Under these circumstances, leakage is likely to occur.
It is usually not difficult to adequately load the sealing material initially. However, in use, the compressive force applied to the sealing material may be reduced as a result of dimensional changes caused by thermal changes, elastic deformation of the parts, or other factors. For example, a captive seal may be initially compressively loaded at ambient temperatures, and thereafter be subjected to cryogenic temperatures down to 450.degree. F or to high temperatures up to 500.degree. F. Under these circumstances, the different coefficients of expansion of the parts of the system may result in a significant loss of compressive pressure in the sealing material.
One effort to solve this problem involves the use of resilient retainers to assist in confining the sealing material. This approach is disclosed, for example, in U.S. Pat. No. 3,572,735 referred to above. The theory underlying use of a resilient retainer is that any relaxation in the sealing material will be "taken up" by expansion of the resilient retainer. Unfortunately, in the patented construction, one of the resilient retainers does not function to take up any relaxation in the sealing material. Although the other of the retainers is resiliently expandable as the sealing material relaxes, it is of intricate wave-like configuration. As such, this latter retainer is difficult and expensive to make.
Another problem occurs when the sealing material expands more rapidly than the walls of the seal cavity in which it is positioned. In this event, there is some danger that the pressure of the expanded sealing material will cause damage to the adjacent parts of the system. In the patented construction, the wave-like retainer is not capable of permitting much, if any, expansion of the sealing material after the sealing material has been compressively loaded.